One goal of modern structural design of masonry buildings and other masonry structures is to ensure that critical structural elements will withstand environmentally induced stresses without, or with limited, structural failure (that is, that the structural elements will perform in a ductile, rather than a brittle, manner). In masonry structures, including for example a plurality of stacked masonry units such as bricks, blocks, or the like bonded together with mortar and having grout and vertical reinforcing steel positioned through openings through the masonry units, critical structural elements include shear walls, columns, and beams. These elements may be subjected to high compressive stresses during earthquakes, wind storms and the like, and are prone to brittle and sudden failures which may, in turn, lead to structural collapse. The purpose of confinement reinforcement is to increase the ductility of masonry structural elements by preventing the rapid, brittle failure of unconfined masonry under compression.
A requirement for masonry confinement reinforcement has been incorporated into the 1988 edition of the Uniform Building Code (UBC) in Section 2412(d) which provides that a "boundary member" is required in the boundaries of shear walls when specified levels of masonry compressive stress will be exceeded. A boundary member must include confinement in the form of #3 bars at a maximum spacing of eight inches or an equivalent thereto.
A brief description of the mechanics of compression failure of masonry units will clarify the function of, and necessity for, confinement reinforcement. When subjected to compressive stresses, unconfined masonry materials respond by shortening in the direction of the compressive stresses and expanding laterally perpendicular to the direction of the compressive stress. A compression failure occurs when the tensile stresses in the masonry structure generated by the lateral expansion of the structure exceed the tensile strength of the grout, the masonry unit outer walls, or faceshell, and/or the bond between the grout and the faceshell, and is characterized by cracks that propagate parallel to the direction of the compressive stress. This type of failure is very brittle, and results in a sudden and substantially complete loss of load-carrying capacity of the failed masonry unit. In masonry structures having vertical reinforcement bars through the spaces, or cells, of masonry units at selected intervals, further loss of structural integrity may occur following the compression failure of the masonry when the vertical reinforcing bars buckle due to the loss of lateral confinement provided by the grout and masonry unit faceshells.
It would thus be desirable to provide masonry confinement reinforcement which is readily installable at the horizontal mortar joints between tiers of masonry units without requiring appreciable change of mortar joint construction to meet applicable cover requirements specified in applicable building codes. Such masonry confinement reinforcement, when properly configured as set forth hereinafter, will withstand and so reduce lateral expansion of the masonry when placed under stress, thus distributing loading and decreasing the tensile stresses in the masonry units and thereby the possibility of brittle failure, and will, after the maximum compressive strength has been reached, act to hold masonry materials together thus limiting the unchecked propagation of cracks in the masonry and the buckling of vertical reinforcing bars.
In this way masonry materials under compression will maintain some residual strength beyond the maximum strength of the materials, thus continuing to absorb energy well beyond the point where an unconfined masonry would have lost all its strength. If compression failure does occur it is thereby confined to small, localized regions. Thus, the overall ductility of the structural materials, and therefore the structure, is improved. Such reinforcement and confinement would meet or exceed the requirements of equivalence to the UBC specified masonry confinement reinforcement discussed above while, by providing a prefabricated unit, resulting in labor, and thus overall cost, savings to builders.